Current methods of matching data rates between networks with independent clock sources are designed to operate in a nearly error free environment. One such method is that described in CCITT Blue Book Recommendations V.110, (1988) which may make clock compensations in fractions of bit times. Since the V.110 frame is comprised of 80 total bits, 48 of which are data bits, if the user clock rate is 4.8 Kb/s then each of the 48 data bits in the V.110 frame are used. If, however, the user data rate is 2.4 Kb/s or 1.2 Kb/s , only 1/2 and 1/4 respectively of the 48 data bits in the V.110 frame are used. In this case, 1/2 and 3/4 of the data bits in the V.110 frame go unused and are eventually redundantly coded. In addition to data bits, clock rate information is also transmitted in the V.110 frame along with network independent clock adjustment information. In error free systems, this information is passed along from one clock source to another so that the independent data source can determine the amount of compensation required to accurately convey data.
The above described method is well suited for integrated services digital network (ISDN) environments where typical bit error rates (BER) are in the order of 10.sup.-9. However, when used in a digital radiotelephone environment, for example the Groupe Special Mobile or GSM digital radiotelephone system environment, the method is subjected to typical BER's on the order of 10.sup.-3 to 10.sup.-5. The implementation of clock rate matching as described by CCITT Recommendations V.110, when used in the GSM environment, causes several problems. First, the method as recommended by the CCITT uses fractions of bit times depending on the data rate to perform clock compensation, however this information is not maintained by the GSM air interface specification which essentially compresses and optimizes the V.110 frame for over-the-air transmission. The lower fractional data rates are lost in the optimization. Second, errors introduced at the GSM air interface may cause the GSM data services to arbitrarily add or delete bits from the user data stream in the V.110 frame. If this occurs, not only are data errors incurred, but also the total number of data bits is disturbed due to corruption of the clock compensation mechanism described in CCITT Recommendation V.110. This problem itself may render certain types of error correcting protocols useless in the GSM environment.
Another shortcoming of the V.110 method of matching user data clock speeds is the requirement of a sampling mechanism to monitor the phase difference between the two clocks in the independent clocks. In order to achieve the required resolution, the clocks should be oversampled in order to measure the required phase difference. This requirement adds an extra and expensive overhead to the GSM data support platform and adds eight complex phase states necessary to implement the compensation mechanism.
Thus, a need exists for a method of matching user data rates with independent clock sources across networks in a high BER environment and also does not require expensive, real-time intensive, and complex additions to the data support platform.